The field of the invention is echocardiography and particularly, the measurement of left ventricular volume using 3D ultrasound data.
Assessment of left ventricular ("LV") function is clinically important. Diastolic and systolic volumes and derived parameters, such as ejection fraction, are generally accepted indicators of LV function. Echocardiography is a widely available clinical method which allows measurement of these parameters. Precision and accuracy of echocardiographic measurements is typically compromised by user's subjectivity, limited windows of access for obtaining echocardiographic scans, and ultrasound signal attenuation and scattering resulting in image artifacts.
In the past decade, compared to single or biplane tomographic techniques that require geometrical assumptions about LV shape, three-dimensional (3D) echocardiography has been shown to provide more precise and accurate evaluation of cardiac volumes. This is particularly relevant in patients with irregular LV geometry, detected during an initial echocardiographic study through multiple tomographic projections. So far, however, the 3D technique has been limited to experimental use due to cumbersome image acquisition (i.e. restricted access windows, long acquisition times, custom probes and special probe holders), time consuming and subjective data processing (i.e. digitization from video tapes and manual delineation of boundaries in numerous serial tomograms), the use of special projections (i.e. parallel or rotational scans with specific incremental steps), and mapping of endo- and epicardium to multiparameter (i.e. computationally intense and subjective for required adjustment of multiple features) models and allowed only limited objective utilization of a priori knowledge about LV geometry.
The concept of 3D ultrasound data acquisition was first described by Baum and Greenwood in 1961, (G. Baum and J. Greenwood, "Orbital Lesion Location by Three-Dimensional Ultrasonography", N.Y. State J Med, 61:4149,4157, 1961). They obtained serial parallel ultrasound images of the human orbit and created a 3D display by stacking together sequential photographic plates bearing the ultrasound images.
Using a transthoracic echocardiographic (TTE) approach, Dekker et al., (D. L. Dekker, R. L. Piziali and D. Dong Jr., "A System For Ultrasonically Imaging The Human Heart In Three Dimensions", Comput. Biomed. Res., 7:544-553, 1974), were among the first to obtain a 3D computer matrix of ECG and respiratory gated image data with a probe attached to a mechanical arm which tracked its spatial motion. Images were subsequently displayed as selected 2D sections. Geiser et al., (E. A. Geiser, L. G. Christie, Jr., D. A. Conetta, C. R. Conit and G. S. Gossman, "A Mechanical Arm For Spatial Registration Of Two-Dimensional Echocardiographic Sections", Cathet. Cardiovasc. Diagn., 8:89-101, 1982), and other investigators have also employed mechanical location systems because of their relative simplicity and low cost.
Ghosh et al., (A. Ghosh, N. C. Nanda and G. Maurer, "Three-Dimensional Reconstruction Of Echocardiographic Images Using The Rotation Method", Ultrasound Med. Biol., 8:655-661, 1982), used a mechanical arm which permits rotation of the transducer only about its imaging axis. Rotation was registered by a precision potentiometer. Although still relying on mechanical parts, this approach introduced a fixed polar coordinate system resulting in a proportionally sampled LV contour in 30.degree. angular increments. Several other investigators have pivoted or rotated the transthoracic transducer at a single location in an effort to obtain tomograms for 3D reconstruction of the LV from a single acceptable acoustic window. The quality of these reconstructions was often limited by the available window and variations in contact of the transducer face with the skin surface during the lengthy acquisition.
Mortiz et al., (W. E. Moritz, and P. L. Shreve, "A Microprocessor-Based Spatial-Locating System For Use With Diagnostic Ultrasound", Pro. IEEE, 64:966-974, 1976), introduced an acoustic position location system ("spark gap"). They employed this device to obtain an arbitrary series of sector scans through the heart which were used to create a computer 3D wire frame model of the LV and calculate its volume (W. E. Moritz, A. S. Pearlman, D. H. McCabe, D. K. Medema, M. E. Ainsworth and M. S. Boles, "An Ultrasonic Technique For Imaging The ventricle In Three-Dimensions And Calculating Its Volume", IEEE Trans. Biomed. Eng., 30:482-492, 1983). King and his coworkers applied this system to guided image acquisition for clinical quantitative evaluation of the LV (D. L. King, M. R. Harrison, D. L. King Jr., A. S. Gopal, R. P. Martin and A. N. DeMaria, "Improved Reproducibility Of Left Arterial And Left Ventricular Measurement By Guided Three-Dimensional Echocardiography", J. Am. Coll. Cardiol., 20:1238-1245, 1992; P. M. Sapin, K. D. Schroder, A. S. Gopal, M. D. Smith, A. N. DeMaria and D. L. King, "Comparison Of Two- And Three-Dimensional Echocardiography With Cineventriculography For Measurement Of Left Ventricular Volume In Patients", J. Am. Coll. Cardiol., 24:1054-1063, 1994; A. S. Gopal, M. J. Schnellbaecher, Z. Shen, O. O. Akinboboye, P. M. Sapin and L. D. King, "Freehand Three-Dimensional Echocardiography For Measurement of Left Ventricular Mass: In Vivo Anatomic Validation Using Explanted Human Hearts", J. Am. Coll. Cardiol., 30:802-810, 1997). Other investigators have also used the spark gap system for calculation of left ventricular volume, mass and for determination of the mitral valve shape. This transducer location technique eliminated the physical connection between the transducer and the reference system thus overcoming some of the restrictions imposed by mechanical transducer location systems. The technique, however, requires a clear line of sight between the transmitting and receiving elements which may not always be feasible. Another disadvantage is that taking tomograms at arbitrary angles may cause unnecessary over-sampling of certain regions and under-sampling elsewhere.
Maehle and coworkers (J. Maehle, K. Bjoernstad, S. Aakhus, H. G. Torp and B. A. J. Angelsen, "Three-Dimensional Echocardiography For Quantitative Left Ventricular Wall Motion Analysis: A Method For Reconstruction Of Endocardial Surface And Evaluation Of Regional Dysfunction", Echocardiography, 11:397-408, 1994), evaluated rotational acquisition system using only 3 or 4 tomograms acquired in polar coordinate. The rest of the LV contour was restored using spline interpolation. This "triplane" or "quadruplane" approach has expectably proven to be superior over single- or biplane methods. The limited number of views makes this method sensitive to foreshortening of the LV apex and may not be sufficient for reproduction of LVs with complicated shape. This work, however, highlighted the fact that a limited number of tomograms greatly reduces an elaborate outlining of an LV contour while still providing valuable information about its function.
The transesophageal echocardiographic (TEE) technique provided a new window to the heart after its clinical introduction in the 1980's (J. B. Seward, B. K. Khandheria, J. K. Oh, M. D. Abel, B. W. Hughes, W. D. Edwards, B. A. Nichols, W. K. Freeman and A. J. Tajik, "Transesophageal Echocardiography: Technique, Anatomic Correlation, Implementation And Clinical Applications", Mayo Clin. Proc., 63:649-680, 1988). Close proximity of the transducer to the heart allowed uniformly higher quality images than those provided by transthoracic echocardiography. Kuroda, Greenleaf, et al. (T. Kuroda, T. M. Kinter, J. B. Seward, H. Yanagi and J. F. Greenleaf, "Accuracy Of Three-Dimensional Volume Measurement Using Biplane Transesophageal Echocardiographic Probe: In vitro Experiment", J. Am. Echo., 4:475-484, 1991) describe 3D LV reconstruction for morphology and volume assessment using a set of sequential 2D tomographic longitudinal images acquired by rotation of the shaft of a transesophageal probe. Martin et al. (R. W. Martin and G. Bashein, "Measurement Of Stroke Volume With Three-Dimensional Transesophageal Ultrasonic Scanning: Comparison With Thermodilution Measurement", Anesthesiology, 70:470-476, 1989; R. W. Martin, G. Bashein M. L. Nessly and F. H. Sheehan, "Methodology For Three-Dimensional Reconstruction Of The Left Ventricle From Transesophageal Echocardiography", Ultrasound Med. Biol., 19:27-38, 1993) have devised an endoscopic micromanipulator for multiplanar transesophageal echocardiographic (TEE) imaging. This system requires a modified transesophageal probe which allows controlled "fan" sweeping with the transducer, thus collecting a pyramidal volume of images. This system capitalizes on an unlimited ultrasound window through the transesophageal access.
Pandian et al. (N. G. Pandian, N. C. Nanda, S. L.
Schwartz, P. Fan, Q. Cao, R. Sanyal, T. Hsu, b. Mumm, H.
Wollschlager, A. Weintraub, "Three-Dimensional And Four-Dimensional Transesophageal Echocardiographic Imaging Of The Heart And Aorta In Humans Using A Computed Tomographic Imaging Probe", Echocardiography 9:677-687, 1992) have evaluated the first commercially available Echo-CT system (Tomtec Imaging Systems, Inc.) for LV function assessment. This PC-based system processes the video signal from an ultrasound scanner and features a model for ECG and respiratory gating. It uses movable holders for TTE probes and a dedicated TEE probe which is made straight and stiff after insertion for an incremental parallel scanning. This system provides dynamic 3D images and can be accommodated to virtually any acquisition geometry and imaging system (N. C. Nanda, R. Sanyal, S. Rosenthal and J. K. Kirklin, "Multiplane Transesophageal Echocardiographic Imaging And Three-Dimensional Reconstruction", Echocardiography, 9:667-676, 1992). However, complicated articulation of the TEE probe and rather bulky mechanical adapters for TTE probes make a routine use of this system difficult.
Variable plane (multiplane) transducers have recently been introduced by all major ultrasonic transducer manufacturers (Hewlett Packard, Acuson, Aloca, Toshiba, etc.). A miniature transducer array is mechanically rotated inside the probe tip to acquire successive tomographic views, which combined form a 3D image data set. Typically, both ECG and respiratory gating are used so that each incremental view is acquired during the same phase of the cardiac and respiratory cycles. This requires several minutes to collect a complete 3D data set and it is difficult to keep the patient motionless and the transducer in proper position.
Another factor contributing to extended scan time is the fact that ultrasound cardiac images are prone to noise, signal dropout, and other artifacts. This and an intrinsic variability of biological shapes make it very difficult to identify heart cavity boundaries without an extensive 3D data set which requires a long time to acquire.
A single application of noise removal and edge detection operators, such as those described by Pitas and Venetsanopoulos (I. Pitas and A. N. Venetsanopoulos, "Order Statistics In digital Image Processing", Proceedings of IEEE 80:1892-1921, 1992) results in sparse information about cardiac boundaries. Additional a priori knowledge about LV shape consuming manual delineation.
Coppini et al. (G. Coppini, R. Poli and G. Valli, "Recovery Of The 3-D Shape of The Left Ventricle From Echocardiographic Images", IEEE Transactions On Medical Imaging 14:301-317, 1995) combine conventional edge detection methods and edge grouping through a curvature analysis with a conventional, two-layer neural network, trained on edge position, direction length, angle, and other local parameters to determine which edge was associated with the LV boundary. Subsequently, an elastic, closed surface ellipsoidal model is used to form the endocardial surface by fitting into the recognized edges. Parametrization of the model represents a significant computer load and requires user-guided adjustment of values controlling flexibility and expansion of the model.
Manhaeghe et al. (C. Manhaeghe, I. Lemahieu, D. Vogelaers and F. Colardyn, "Automatic Initial Estimation Of The Left Ventricular Myocardial Midwall In Emission tomograms Using Kohonen Maps", IEEE Transactions on Pattern Analysis and Machine Intelligence 16:259-266, 1994) use self-organizing maps (SOM) to delineate myocardial midwall in emission tomograms. The SOM is an unsupervised neural network technique, introduced by T. Kohonen (T. Kohonen, "Self-Organizing Maps", Springer-Verlag, N.Y., 1995). The SOM consists of nodes that distribute themselves according to intensities in the emission images. Connection of neighboring nodes results in a closed outline of the myocardial midwall.